1. Field of the Invention
The present invention relates to an analog switching circuit and a gradation selector circuit, particularly relates to an analog switching circuit and a gradation selector circuit used for selectively outputting one of plural analog voltage values.
2. Description of the Related Art
A gradation selector circuit that selectively outputs one of plural analog voltage values according to a control signal is known. The conventional type gradation selector circuit will be described below.
FIG. 6 is a circuit diagram showing the configuration of the conventional type gradation selector circuit. The conventional type gradation selector circuit 110 is provided with a resistor string circuit (a ladder resistor circuit) 114 and a selector circuit 111.
The resistor string circuit 114 is formed by connecting resistive elements R in series between a high potential power source and a low potential power source.
The selector circuit 111 includes plural analog switching circuits 111a, 111b, 111c, - - - each of which is connected between a node of each resistive element R of the resistor string circuit 114 and the output terminal of the gradation selector circuit 110.
In the gradation selector circuit 110, the node of each resistive element R of the resistor string circuit 114 has analog voltage acquired by dividing voltage between the high potential power source and the low potential power source by the number of the resistive elements R.
In the selector circuit 111, only one of the plural analog switching circuits 111a, 111b, 111c, - - - is selected and is turned on according to input data (an input control signal). Analog voltage applied to the corresponding node of the selected analog switching circuit 111x (x=a, b, c, - - - ) is output from the output terminal of the gradation selector circuit 110.
In the conventional type gradation selector circuit 110, for the analog switching circuit, for example, the analog switching circuits 111a, 111b, 111c, - - - in each of which an N-type insulated gate field effect transistor (hereinafter called an N-type MOS transistor) and a P-type insulated gate field effect transistor (hereinafter called a P-type MOS transistor) are connected in parallel are used.
In the prior art shown in FIG. 6, potential difference is made between the potential of a source electrode which is one electrode of the source and the drain of each transistor in the analog switching circuit 111a for example and the potential of a back gate electrode. Therefore, the threshold voltage of the MOS transistor increases by back gate effect. As the voltage of the analog switching circuit 111a, - - - approaches analog voltage in the vicinity of intermediate potential, ON-state resistance increases, acting speed is slowed and an error among voltage output from the output terminal of the gradation selector circuit 110 is increased.
To avoid the problem, there are a method of controlling a threshold and a method of changing the size of a transistor, however, in both cases, the cost of the chip is increased.
For related technique, the technique of an analog switch is disclosed in Japanese published unexamined patent application No. 2000-77992. FIG. 7 is a circuit diagram showing the configuration of one example of an analog switch 120 used for a gradation selector circuit disclosed in the patent application.
In an analog switch 121, an N-type MOS transistor 123, an N-type MOS transistor 125 and an N-type MOS transistor 127 are connected. The N-type MOS transistor 123 is connected between a first terminal and a second terminal. The N-type MOS transistor 125 is connected between the first terminal and the back gate electrode of the N-type MOS transistor 123. The N-type MOS transistor 127 is connected between the back gate electrode of the N-type MOS transistor 123 and a low potential power source.
In an analog switch 122, a P-type MOS transistor 124, a P-type MOS transistor 126 and a P-type MOS transistor 128 are connected. The P-type MOS transistor 124 is connected between a first terminal and a second terminal. The P-type MOS transistor 126 is connected between the first terminal and the back gate electrode of the P-type MOS transistor 124. The P-type MOS transistor 128 is connected between the back gate electrode of the P-type MOS transistor 124 and a high potential power source.
These two analog switches 121, 122 are connected in parallel and compose the analog switch 120 shown in FIG. 7.
The first terminal is connected to the resistor string circuit (the ladder resistor circuit) 114 shown in FIG. 6 and the second terminal is connected to the output terminal.
In case the analog switch 120 is turned on, the source electrode of each transistor and the back gate electrode of each transistor are at the same potential via the N-type MOS transistor 125 and the P-type MOS transistor 126. Therefore, the increase of threshold voltage by back gate effect can be inhibited. The ON-state resistance of each transistor corresponding to analog voltage in the vicinity of intermediate voltage can be reduced.
Besides, in case the analog switch 120 is turned off, leakage current when the first terminal and the second terminal are at different potential can be inhibited because the back gate electrode of each transistor is connected to the high potential power source and the low potential power source via the N-type MOS transistor 127 and the P-type MOS transistor 128.
The acting speed is accelerated and the precision of output voltage can be enhanced by adopting the configuration described above.
In this technique, in a process in which the threshold voltage of a transistor is set to a high value or in case the use at low voltage is required, a case that the N-type MOS transistor 125 or the P-type MOS transistor 126 cannot be fully turned on occurs. As a result, the source electrode which is one electrode of the source and the drain of the N-type MOS transistor 123 or the P-type MOS transistor 124 and the back gate electrode of the same transistor are not at the same potential and as a result, a case that the ON-state resistance of the analog switch 120 never drops occurs.